The Condor 113(3) - University of Northern British Columbia

The Condor 113(3):555–564
 The Cooper Ornithological Society 2011
CORRELATES OF DEUTERIUM (δD) ENRICHMENT IN THE FEATHERS OF ADULT
AMERICAN KESTRELS OF KNOWN ORIGIN
Jennifer L. Greenwood1
and
Russell D. Dawson
Ecosystem Science and Management Program, University of Northern British Columbia, 3333 University Way, Prince George,
British Columbia V2N 4Z9, Canada
Abstract. Many populations of the American Kestrel (Falco sparverius) are in decline; in northern populations it is unknown whether low recruitment and rates of return are due to mortality, poor detection, or dispersal.
Ratios of stable hydrogen isotopes in feathers (δDf ) have been used widely to estimate origins of birds, in some
cases providing estimates of adults’ dispersal between breeding seasons and post-natal dispersal of young. We attempted to use δD in feathers, grown during breeding, to identify returning and immigrant birds and to quantify
dispersal. We used birds of known origin to establish the expected local δDf but found a high degree of deuterium
enrichment in these individuals relative to the local δD of rainfall (δDp) and a significant difference between δDf of
nestlings and adults, which complicated the distinguishing of local birds from immigrants. We subsequently compared the δD of primary feathers and plasma of adults and tested for relationships among the δD of adult feathers
and adult mass, body size, and reproductive effort at the time of growth to explore the cause of deuterium enrichment in adult feathers. Adults’ feathers were significantly more deuterium-enriched than plasma, and their distribution did not overlap with that of nestlings’ feathers. Larger males and females that fledged female nestlings of
greater mass had feathers that were more deuterium-enriched, while males whose mates laid clutches of greater
volume had less enriched feathers. We discuss our results with respect to the prevailing hypotheses for deuterium
enrichment of raptor feathers, particularly the evaporative-cooling hypothesis.
Key words: δD, American Kestrel, body size, Falco sparverius, feathers, enrichment, reproductive effort,
stable hydrogen isotopes.
Correlatos de Enriquecimiento de Deuterio (δD) en las Plumas de Individuos Adultos de
Falco sparverius de Origen Conocido
Resumen. Las poblaciones del halcón Falco sparverius están disminuyendo en muchas partes y en aquellas
del norte no se sabe si el bajo reclutamiento y las tasas bajas de retorno se deben a mortalidad, a deficiente detección o a dispersión. Las proporciones de isótopos de hidrógeno estables en las plumas (δDf ) se han utilizado ampliamente para estimar el origen de las aves, en algunos casos proporcionando estimaciones de dispersión de adultos
y jóvenes. Intentamos utilizar el δD que se encuentra en las plumas que crecieron durante la época de cría para
identificar a las aves que regresaron y a las que inmigraron, así como para cuantificar la dispersión. Utilizamos
aves de origen conocido para establecer la cantidad de δDf local esperada. Sin embargo, encontramos un alto grado
de enriquecimiento de deuterio en estos individuos con relación al δD local proveniente de las precipitaciones
(δDp) y una diferencia significativa entre el δDf de los pichones y de los adultos, lo que complicó la distinción de las
aves locales de las inmigrantes. Posteriormente, comparamos el δD de las plumas primarias y del plasma de adultos y pusimos a prueba la existencia de relaciones entre el δD de las plumas de los adultos y su peso, tamaño corporal y esfuerzo reproductivo en el momento del crecimiento de las plumas para explorar la causa de enriquecimiento
de deuterio en las plumas de un adulto. En las plumas de adultos, el δD fue significativamente mayor que en el
plasma y su distribución no se superpuso con la de las plumas de los pichones. Las plumas de los machos y de las
hembras de mayor tamaño, que produjeron polluelos volantones hembra de mayor peso, estuvieron enriquecidas
con deuterio, mientras que los machos cuyas parejas tuvieron puestas de mayor volumen tuvieron plumas menos
enriquecidas. Discutimos nuestros resultados en el contexto de las hipótesis vigentes para el enriquecimiento con
deuterio de plumas de aves rapaces, con especial énfasis en la hipótesis de evaporación y enfriamiento.
INTRODUCTION
Studies of avian population dynamics are likely to be incomplete without estimates of adults’ movements from one site
of breeding to the next and juveniles’ movements from their
natal site to their site of first breeding (Negro et al. 1997, Walters 2000). Movement among breeding populations regulates
gene flow and resulting population genetic structure, and the
extent to which an individual moves from year to year will influence its familiarity with the local environment for breeding
Manuscript received 29 October 2010; accepted 4 April 2011.
1
E-mail: [email protected]
The Condor, Vol. 113, Number 3, pages 555–564. ISSN 0010-5422, electronic ISSN 1938-5422.  2011 by The Cooper Ornithological Society. All rights reserved. Please direct all
requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/
reprintInfo.asp. DOI: 10.1525/cond.2011.100213
555
556 JENNIFER L. GREENWOOD
and
RUSSELL D. DAWSON
(Danchin and Cam 2002, Marr et al. 2002). Both of these
components have direct consequences for individual fitness
(Doligez and Pärt 2008); therefore, accurate estimates of recruitment rates and the scale of movement among breeding
populations are important for determining population status
and source–sink dynamics. Nonetheless, for species that are
not easily tracked, it can be difficult to determine whether dispersal constitutes a small-scale movement to an adjacent area
or a large, landscape-scale movement. Extrinsic markers are
useful for determining rates of return to a given study area,
but when recapture rates are low, it is difficult or impossible to
ascertain whether low return rates are due to mortality or emigration (Waser et al. 1994, Rubenstein and Hobson 2004).
A growing number of studies have used stable hydrogen
isotopes to estimate the origins of birds. Hydrogen-isotope ratios are the ratio of heavy (deuterium) to light hydrogen (2H/1H,
or δD); precipitation generally becomes more depleted in deuterium (2H), i.e., values of δD become more negative, from
southeast to northwest in North America and from low to high
altitudes (Dansgaard 1964). Consumers incorporate the local
isotopic signature of rainfall into their tissues and although a
host of variance-generating processes introduce error into the
analysis of isotope-ratios in tissues (Wunder and Norris 2008),
spatial patterns in isotopic variation are generally reflected in
the tissues of higher-order organisms (Chamberlain et al. 1997,
Hobson and Wassenaar 1997). When synthesized from exogenous sources, tissues that are metabolically inert after growth
(claws, feathers) should reflect only the isotopic signature of the
food and water being ingested at the time of growth (Hobson
and Clark 1992, Hobson and Wassenaar 1997).
In numerous locations throughout North America, the
population of the American Kestrel (Falco sparverius) is undergoing an unexplained decline (Smallwood et al. 2009).
The kestrel’s recruitment and rates of return to breeding areas are generally low (Smallwood and Bird 2002, but see
Balgooyen 1976) so its movement within and among years is
difficult to describe. In the population we studied, located in
north-central Saskatchewan, kestrels molt their primary flight
feathers once per year, during breeding (Smallwood and Bird
2002). Therefore, the δD of primaries collected before molt in
the current breeding season should reflect the approximate location of growth in the preceding nesting season. We designed
a study in which we used the δD of feathers (δDf ) to estimate
these kestrels’ dispersal between site of breeding or hatching
and site of next breeding. Our study extended over 3 years,
allowing for the collection of feathers from recaptured birds
known to breed in, or hatch at, our location in the previous
year. We intended for these samples of known origin to allow
us to establish the baseline “local” δDf, which we expected to
differ more from the long-term amount-weighted δD averages
of local growing-season precipitation (δDp) than in other orders of birds (Meehan et al. 2003, Smith and Dufty 2005). The
distribution of δDf derived from our local individuals should
allow us to estimate the probability that each individual of unknown origin had immigrated to or had bred (or was hatched)
at our location in the previous breeding season. This information would facilitate much-needed estimates of dispersal and
recruitment and provide a basis for the evaluation of the influence of dispersal strategy on reproductive success.
Indeed, the feathers of known-origin adults were substantially enriched in deuterium in comparison with long-term
averages of local δDp , weighted by amount of precipitation, as
well as the expected value of δDf derived from a kestrel-specific
geospatial predictive model (Hobson et al. 2009; see Results).
In the absence of an understanding of the proximate cause and
geographic variation of this enrichment, we therefore pursued
exploratory analyses to shed light on the processes responsible
for this phenomenon.
Meehan et al. (2003) and Smith and Dufty (2005) found
similar deuterium enrichment in the feathers of adult accipiters.
Raptors molt over extended periods, which usually overlap with
breeding. If physiological processes during breeding influence
the δD within the body, these changes would be evident in the
δD of tissues such as feathers. Meehan et al. (2003) proposed
three nonexclusive hypotheses to explain deuterium enrichment
in raptor feathers. First, if raptors consume migrant birds, the
isotope ratios incorporated from tissue of the prey should reflect
the more positive δD values found in the more southern locations from which these prey originated or passed through. Second, among migratory raptors, new feathers may be produced
from tissue reserves derived in the nonbreeding season. Third,
body water becomes deuterium-enriched when the lighter form
of hydrogen is differentially lost through evaporative water loss
as a result of the energetic demands of breeding (Wolf and Martinez del Rio 2000, Meehan et al. 2003, McKechnie et al. 2004).
Reproduction and molt are both energetically expensive processes (Dietz et al.1992, Weiner 1992), and, at such times, when
the metabolic heat load is high, heat balance is often maintained
via evaporative water loss (Kvist et al. 1998).
We analyzed the stable hydrogen isotopes in feathers and
plasma (collected during the period of feather growth) of adults
and in feathers of nestlings to examine the cause of deuterium
enrichment of feathers in our population. Feather keratin is
synthesized from amino acids transported in the blood; upon
completion of a feather’s growth, it becomes metabolically inert. Although a general lack of knowledge about tissue-specific
discrimination factors for hydrogen isotopes complicates predictions, we expected that if kestrels mobilize tissue reserves
from the nonbreeding season or consume migrant prey during
feather synthesis, we should find the δD values of plasma (i.e.,
protein derived from endogenous reserves or migrant prey) to
be more similar to those of their own feathers than to the δD
values of local precipitation or those of the feathers of nestlings
provisioned with local prey. In addition, we examined the relationships between δDf and measures of body mass and size and
of reproduction in the previous year; these are both indicators
DEUTERIUM ENRICHMENT IN THE AMERICAN KESTREL 557
of conditions with the potential to influence energetic and
physiological processes experienced by the birds at the time of
feather growth. The activities involved in reproduction elevate
metabolism and heat production, and thermal conductance decreases exponentially with increasing body mass (Schleucher
and Withers 2001), which may further constrain the capacity
for heat dissipation of larger animals in comparison to smaller
ones. Therefore, we predicted that if deuterium enrichment is a
reflection of evaporative cooling that occurs as a function of the
energetic rigors of breeding, the degree of enrichment should
vary as a function of body mass and/or size and reproductive effort in the preceding breeding (molting) season. We expected a
greater degree of enrichment among birds that initiated breeding earlier if feather deuterium enrichment were a function of
migrant prey but expected little relationship between size or
mass and deuterium enrichment. We made no predictions with
respect to endogenous reserves and feather enrichment because
of the potential for interaction with a host of other factors, including seasonal interactions influencing individual condition,
mate choice, and territory quality.
Methods
Our study area was located near Besnard Lake, Saskatchewan (55° N, 106° W), in the boreal forest. The American Kestrel, a small, migratory falcon, is a secondary cavity-nester
and readily uses nest boxes. We used approximately 255 nest
boxes mounted an average of 3.6 m above ground on trees and
decommissioned power poles. These were distributed along
a network of gravel roads among a variety of forest-structure
types. Our field work extended from April to July in 2007,
2008, and 2009.
Tissue-sample collection and body size
of adults
In 2007 and 2008 we captured adult kestrels with bal-chatri
traps (Berger and Mueller 1959) prior to laying; in all three
years we captured them by hand in the nest box during incubation. All birds captured in 2007 were of unknown origin (i.e., unknown location of breeding or hatching in 2006),
whereas in 2008 and 2009 we captured a subsample of kestrels
that had been sampled at our study area in the previous years.
These recaptured birds constituted our sample of individuals
of known origin; the feathers we collected from these birds
were grown at our study location in the preceding breeding
season. For each bird, we measured the 10th primary, central
and outer rectrices, and wing chord to the nearest 0.5 mm with
a ruler. We measured tarsus length to the nearest 0.1 mm with
digital calipers and mass to the nearest gram with a spring
balance. We collected feathers for δD analysis by clipping approximately 1.5 cm of the vane from the distal end of the 4th
primary on the right wing. If this feather was already molted,
we sampled from the left wing or clipped the 5th primary if the
4th was molted from both sides. The kestrel’s molt begins with
the 4th primary and proceeds in both directions (Smallwood
and Bird 2002); at our study area this begins during breeding,
usually during incubation. We used this first-molted feather
to ensure we obtained one that was grown at the location of
breeding. Feathers were stored at room temperature in paper
envelopes. We collected a small blood sample (~150 μL) from
the brachial vein of each adult with a 27-gauge needle and
syringe or microcapillary tube. Blood was stored on ice until
it was centrifuged; plasma was then drawn off and stored frozen at –20°C until isotope analysis.
To distinguish between the effects of structural size and
tissue mass on isotopic composition in the feathers, we first
calculated an index of structural size by principal components analysis (PCA; Bortolotti and Iko 1992). The five linear measurements of adults’ size (first capture only) were
entered in PCA, each year and sex analyzed separately. These
analyses explained between 52% and 62% of the variation,
and we used the first component (PC1) as a measure of structural size. Variation in PC1 was driven largely by wing chord,
which loaded strongly and positively in all cases (0.81–0.90).
For males, we then used the residuals from a linear regression
of body mass on PC1 (F1,76 = 9.40, P = 0.003) as a measure of
body mass not attributable to structural size (residual mass;
Dawson and Bortolotti 2000). There was no relationship between mass and PC1 among females in our study area (F1,98 =
0.002, P = 0.97), so we used raw mass for females. We used
the mass of birds captured during incubation, except for cases
where birds were captured only with bal-chatri traps and not
subsequently in a nest box. The condition of a kestrel can vary
through the breeding season (Dawson and Bortolotti 1997),
but we detected no linear or polynomial relationships between
capture date or clutch-initiation date and raw or residual mass
for either sex so did not correct for capture date.
Reproduction
Starting in early May, we visited nest boxes every 3 to 5 days
to determine clutch-initiation dates. Upon completion of a
clutch, we returned to determine its size, measure eggs, and
capture adults. We measured the length (l) and breadth (b)
of each egg to the nearest 0.1mm with digital calipers. From
these measurements, we calculated the clutch’s total volume summing the volume of each egg in the nest, given by
0.51lb2 (Hoyt 1979). In our area, most kestrels lay clutches of
4 or 5 eggs (Tella et al. 2000), so we examined clutch volume
because it is more variable than clutch size.
We aged nestlings on the basis of the date of hatching of
the first nestling (day 0) in each nest, determined by visiting
the nest every day, starting 1 to 2 days before the predicted
hatch date. We monitored nests throughout brood rearing,
and when nestlings were 24 days old we measured length of
the 10th primary and mass of each nestling. In 2009, we also
clipped the distal 1.5 cm of the 4th primary on the right wing
of one male and one female nestling from each nest. These
558 JENNIFER L. GREENWOOD
and
RUSSELL D. DAWSON
feathers were stored in paper envelopes until processed in the
lab. The kestrel’s sexual dimorphism in plumage and size is
evident by 12 days of age (Anderson et al. 1993); we calculated the mean mass and length of the 10th primary of nestlings in each nest at day 24 for males and females separately.
Nestlings fledge between 24 and 30 days of age, so we did
not visit nests during this period to avoid premature fledging.
If we found no nestlings dead in the box 35 to 40 days after
hatching, we considered the number of nestlings in the box at
day 24 to be the number successfully fledged.
Isotope preparation and laboratory
analysis
Each plasma sample was freeze-dried and stored in small
tubes (Wassenaar 2008). Feathers were cleaned of surface
oils by an overnight soak and subsequent rinse with a 2:1
chloroform:methanol solvent mixture. To avoid contamination, we cleaned dirty feathers (e.g., nestling feathers caked in
feces) with ultra-pure water prior to the solvent rinse. Feathers were then left in a fume hood to air dry for at least 48
hr. We clipped small portions of vane from the distal tip of
each feather for analysis; for those feathers sampled more than
once (repetitions among auto-runs) we clipped vane immediately proximal to the prior clipping or from the most distal
portion on the other side of the rachis. For all tissue types, we
loaded 350 ± 10 μg of prepared sample into silver capsules
(Model D2002, Elemental Microanalysis, Okehampton, UK),
then crushed them to remove all air.
We analyzed stable hydrogen isotopes at the Stable Isotope Hydrology and Ecology Laboratory at the National Water Research Institute (Environment Canada, Saskatoon,
Saskatchewan) by the comparative equilibration approach detailed by Wassenaar and Hobson (2003); this method uses precalibrated keratin reference standards to correct for the portion
of exchangeable hydrogen in keratinous tissue. We pyrolyzed
samples at high temperatures by continuous-flow isotope-ratio mass spectrometry to determine the ratio of nonexchangeable 2H/1H (deuterium, δD) in the resulting H2 gas (Wassenaar
and Hobson 2003). We express δD results per mil (‰) relative
to Vienna Standard Mean Ocean Water (VSMOW-SLAP),
given by the formula
δ = [(2H/1Hsample)/(2H/1Hvsmow) – 1] × 1000.
Wassenaar and Hobson (2006) reported the repeatability
of measurement of keratin reference standards as better than ±
2.0‰. Our samples were analyzed for hydrogen-isotope composition in three separate years (2007–2009); known-origin samples (birds recaptured; feathers grown in 2007 and 2008) were
analyzed in 2008 and 2009 only. Previous studies have suggested variation in the reproducibility of measurements among
auto-runs (Lott and Smith 2006, Smith et al. 2009); therefore,
we included repeat samples in each auto-run to account for
temporal bias in our isotope measurements. We incorporated
seven repeats in each auto-run in 2008 and 2009; the mean
difference between pairs of auto-runs ranged in magnitude
from –0.17‰ to 21.17‰, and the standard deviation of these
differences ranged from 5.25‰ to 10.89‰ (mean SD: 7.92‰).
We conducted reduced major-axis regression between repeats
from one arbitrarily chosen auto-run and each additional set of
repeats. These correction equations allowed us to normalize δD
values to one common auto-run. Our repeat samples do not represent true replicates because δD can vary along the length of a
feather (Smith et al. 2008); however, all feather samples, including repeats, were taken from the distal 1.5 cm of the feather to
minimize this possibility. We did not include replicates in 2007
samples (feathers grown in 2006), so we could not correct these
to one auto-run; therefore, we corrected samples of known origin only (grown in 2007 and later) to the same run.
We obtained the predicted mean annual and mean growing-season amount-weighted δD of precipitation (δDp) for our
study site from the Bowen and Revenaugh (2003) and Bowen
et al. (2005) spatial models (Bowen 2010). For this we used
coordinates of a point near the center of the study area (55°
12.9248′ N, 106° 03.0807′ W). The growing season encompasses those months with a mean temperature greater than 0
°C. We obtained the expected δDf of kestrels at our location
from the kestrel-specific surface in Hobson et al. (2009), which
depicts the geographic distribution of expected δDf from a reduced major-axis regression between feathers of juvenile kestrels of known origin (Lott and Smith 2006) and the GIS model
of expected growing-season δDp (Bowen et al. 2005).
Statistical analyses
We inspected the raw δDf values from all birds (known
and unknown origin) prior to analysis. We plotted the autorun-normalized values for the δDf of known local adults (recaptures in 2008 and 2009) and nestlings of 2009 and the δD of the
plasma samples against the mean annual and mean growingseason amount-weighted δDp and the kestrel-specific expected
δDf for our area. We did not test for statistical differences between the δDf of adults and nestlings because these feathers
were grown in different years. We used a paired t-test to examine differences between δD of feathers and plasma collected
from the same individual; plasma was sampled in the year
feathers were grown (the year preceding feather collection). All
subsequent analyses examining relationships between δDf and
adult characteristics were carried out with only those birds that
had been caught in our study area in the previous year (body
size and mass) and were known to breed there in the previous
year (reproduction). For each of these analyses, we used the
morphological measurements and reproductive traits recorded
in the year prior to feather collection, as these were the conditions experienced during the time of feather synthesis.
To determine whether all data met distributional assumptions, we examined plots of residuals and tested for normality
with Shapiro-Wilk’s test; we assessed equality of group variance with Levene’s test. We analyzed males and females separately and used a general linear model to test for relationships
DEUTERIUM ENRICHMENT IN THE AMERICAN KESTREL 559
between δDf and structural size and between δDf and residual
mass of known local males. Structural size (PC1), residual
mass, and year were fixed factors, and we included terms for
interactions between year and both structural size and residual mass. A number of females were caught in multiple years,
so we included female identity as a random factor in a linear mixed model to control for the nonindependence of these
observations. Structural size (PC1), raw mass, and year were
fixed factors, and we included terms for interactions between
year and both structural size and mass. For both males and
females, we removed terms sequentially where P ≥ 0.10. We
used all known local birds that were captured in the study area
at any time during the previous breeding season.
To explore possible relationships between δDf and reproductive effort during the time of feather growth, we calculated
Pearson’s correlations between δDf and variables associated
with each adult in the previous year that may represent some
measure of the amount of energy expended during breeding.
These included clutch-initiation date, total clutch volume, sex
ratio of the brood at day 24, number of young fledged at each
nest, mean mass per nest of male and female nestlings at day
24, and mean length of the 10th primary of males and females in
each nest at day 24. We chose correlations because of the exploratory nature of this study and because the likely co-variation
of reproductive variables with other factors (e.g., resource
availability, adult quality) may render some models too complex for the scope of our data.
Figure 1. Frequency distribution of raw values of stable hydrogen isotopes for the primaries (δDf ) of all American Kestrels (n =
262) sampled in 2007 and 2008. Mean growing-season δD weighted
by amount of precipitation (δDMGS) is indicated with an arrow.
We considered results significant at P ≤ 0.05 and present
means ± 1 SE unless otherwise indicated. All statistical analyses were done in SPSS 16 (SPSS, Inc., Somers, NY).
Results
Tissue–isotope distribution
For all adults sampled in 2007 and 2008, the modal δDf was
–62‰ (minimum = –134‰, maximum = –20‰, SD = 26.20‰).
The frequency distribution of δDf in adult kestrels was bimodal,
with a break at approximately –90‰ (Fig. 1). The subsample
of kestrels of known origin (those sampled in our study area in
consecutive years) had a mean δDf well above both the mean
annual (78.89‰ enriched), and mean growing-season (63.93‰
enriched) δDp and expected δDf (on the basis of Hobson et al.
2009) for this location (93.00‰ enriched; Fig. 2). In contrast,
the mean δDf of nestling kestrels was significantly less enriched
relative to expected values (mean annual δDp: 18.35‰ enriched; mean growing-season δDp: 3.39‰ enriched; expected
δDf: 31.76‰ enriched); distributions of δDf in nestlings and
adults did not overlap (Fig. 2). Among the adults from which we
analyzed both plasma and feathers, δDf was significantly more
positive than δD of plasma (paired t-test: t9 = 14.03, P < 0.001);
the mean difference was 93.41‰ ± 6.66 SE (Fig. 2).
Figure 2. Values of stable hydrogen isotopes (δD) for the feathers (AF) and plasma (AP) of adult American Kestrels of known origin recaptured and sampled in 2008 and 2009 and for the feathers of
nestling kestrels (NF) sampled in 2009. All isotope values are normalized to a common auto-run. Reference lines indicate the mean
growing-season δDp weighted by average precipitation (solid line;
Bowen et al. 2005) and expected δDf derived from a GIS-based
model of feathers of juvenile kestrels of known origins (broken line;
Hobson et al. 2009) for our study area. Sample sizes are in brackets
above each set of observations.
560 JENNIFER L. GREENWOOD
and
RUSSELL D. DAWSON
Deuterium enrichment, size, and mass
We found no significant relationship between δDf and either size (PC1: F1,11.8 = 0.34, P = 0.57 ) or mass (F1,11.90 = 2.30,
P = 0.64) of females, but there was a significant year effect
(F1,6.25 = 11.36, P = 0.01). Nonetheless, exploratory analyses
showed a negative correlation between δDf and mass in 2008
(r = –0.63) that was not significant (P = 0.18) but perhaps notable in light of the small sample size in that instance (n = 6).
For males, there was a significant effect of size (F1,12 = 11.58,
P = 0.005; Fig. 3), suggesting that deuterium enrichment was
greater in larger-bodied males . There was no effect of residual mass (P = 0.27) or year (P = 0.97), so we removed these
factors from the model.
Deuterium enrichment and reproduction
Correlation analysis showed a negative relationship between
clutch volume and the δDf of male parents (Table 1), suggesting that the feathers of males whose mates laid clutches of
larger volume were less deuterium enriched. In addition, the
correlation coefficient between length of the 10th primary of
male nestlings and δDf of male parents was –0.78; this relationship was not significant (P = 0.07), but sample size was
small (n = 6). Among the variables we tested, only the mean
mass of female nestlings at day 24 showed a significant positive relationship with δDf of adult females (Table 1), indicating that adult females that raised female young fledging with a
higher mass experienced greater deuterium enrichment.
Discussion
δDf and estimating dispersal
The feathers of adult kestrels of known breeding origin showed
a degree of deuterium enrichment relative to weighted average growing-season δDp (Bowen et al. 2005) and expected δDf
(Hobson et al. 2009) that is in clear agreement with the results
of Meehan et al. (2003) and Smith and Dufty (2005). These
results provide evidence that this phenomenon is not limited
to accipiters but extends to falcons and perhaps to other birds
with similar life histories. In the absence of the knowledge that
each of these individuals bred at our study area in the previous
year, the δDf values of our subsample (–74.73 to –22.95‰) of
kestrels implies an origin much farther to the south and/or east.
Our interpretation of the greater data set would likely have
been that our sampled population comprises both birds that
bred at our site in the previous year and birds that dispersed
from other locations (immigrants). Nestlings’ values were
less deuterium enriched relative to both expected δDp and δDf.
The distribution of δDf of nestlings (Fig. 2) versus adults of
known origin suggests that the bimodal distribution of δDf of
all sampled individuals (Fig. 1) is likely a function of the combination of second-year (values less than -90‰) and after-second-year (values above -90‰) birds in the sample rather than
variation in site of breeding. Both GIS models of expected δDf
(Lott and Smith 2006, Hobson et al. 2009) were constructed
from juvenile feathers of known origin, so, given the difference in δDf we observed between nestlings and adults, it is not
surprising that our δDf values in adults of known origin were
deuterium-enriched relative to that predicted from Hobson et
al. (2009). Although the use of feathers from birds of known
origin allowed us to establish a “local” δDf distribution, we
were unable to use this distribution to estimate the probability
TABLE 1. Pearson’s correlations for the relationship between the
ratio of stable hydrogen isotopes in the 4th primary of adult American Kestrels (δDf ) and reproduction in the year the feather was
grown.
Male
Variable
Figure 3. The relationship between structural size (PC1) and
δDf of male American Kestrels originating at our study location near
Besnard Lake, Saskatchewan.
Clutch-initiation date
Clutch volume
Sex ratio
Number of young fledged
Mean mass of female
nestlings
Mean mass of male
nestlings
Mean length of p10 of
female nestlings
Mean length of p10 of
male nestlings
Female
r
P
n
r
–0.37
–0.84
0.40
–0.58
0.53
0.47
0.04
0.43
0.23
0.36
6
6
6
6
5
0.06
0.07
–0.03
0.31
0.82
P
n
0.21
0.69
6
0.29
0.41
10
0.42
0.48
5
–0.25
0.56
8
–0.78
0.07
6
–0.32
0.37
10
0.86 12
0.81 13
0.94 11
0.30 13
0.01
8
DEUTERIUM ENRICHMENT IN THE AMERICAN KESTREL 561
that birds of unknown origin were either local or outside recruits, as a result of the difference between known δDf values of nestlings and adults. Extensive observation at our study
area through time (G. R. Bortolotti, R. D. Dawson, and J. L.
Greenwood, unpubl. data) has shown that adult birds cannot
be reliably aged. Therefore, we would be unable to determine
whether individuals of unknown origin should be compared
to our distribution of local nestlings or adults.
Our results reinforce assertions (Meehan et al. 2003,
Smith and Dufty 2005) that until such time as the physiological underpinnings of deuterium enrichment in feathers of adult
raptors are understood, hydrogen isotopes should be used with
caution to estimate origins of raptors. Hobson et al. (2009) explored the utility of using the relationship between δ18O and
δD to identify kestrel feathers that were unreliable indicators
of origin, showing that those feathers with δD above approximately –20‰ departed from the generally linear relationship
with δ18O. However, their study did not sample birds of known
origin, so although the authors recommend using δ18O as a
tool for eliminating birds with enriched values from interpretation, it is yet to be tested whether the departure of the
δ18O–to-δD relationship from linearity is directly related to
deuterium enrichment relative to local rainfall and whether
this would be reliable for all sampled populations.
Tissue sources and deuterium enrichment
Together, the δD values of plasma and feathers of adults neither support nor disprove the first or second hypothesis of
Meehan et al. (2003) as explanations for deuterium enrichment in feathers of adult kestrels. Tissue-specific discrimination occurs with other stable isotopes (e.g., DeMots et al. 2010)
and may occur with δD also. Nonetheless, values for plasma
were, on average, 93.41‰ more depleted than those for feathers of the same individuals, and they were more depleted than
long-term weighted averages of both mean annual and mean
growing-season δDp. If kestrels were mobilizing endogenous
tissue reserves or consuming migrant prey whose δD values
reflected a more southern origin, it is possible that the plasma
of adults would be more positive than the δD of long-term precipitation averages. Furthermore, at our study location birds
constitute a smaller part of the kestrel’s breeding-season diet
than do small mammals, insects, and frogs combined (Bortolotti et al. 2000). Experiments designed to examine tissuespecific discrimination in this species would undoubtedly
clarify predictions associated with these two hypotheses.
Unfortunately, comparisons between plasma and feathers were less useful for evaluating the evaporative-cooling
hypothesis. Preparation for isotopic analysis requires the removal of water from the plasma. Water-bound hydrogen
in plasma is the most likely source of light hydrogen lost to
evaporation (Schoeller et al. 1986), and body water supplies
approximately 26 to 32% of nonexchangeable hydrogen in
feather keratin (Hobson et al. 1999). It is unknown whether the
deuterium enrichment of this water as a result of evaporative
cooling is sufficient to produce such enriched values in feathers, but, given our results, this possibility warrants further
examination.
δDf relationships
Our results relating deuterium enrichment of adult feathers to
body mass, size, and reproduction are difficult to interpret in
the context of the first and second hypotheses of Meehan et al.
(2003). Data on a host of factors, including but not limited to
the relative abundance of various prey sources, annual variation in weather that may influence migrant prey or the behavior of kestrels, and the influence of carryover effects from the
previous season on body condition are necessary for these hypotheses to be evaluated better. Our results concerning adult
mass and size and reproduction do, however, provide indirect
support for the evaporative-cooling hypothesis.
Birds tend to have basal metabolic rates higher than those
of similar-sized mammals (Arieli et al. 2002), and the majority of metabolic energy is converted to heat rather than to mechanical work (St. Laurent and Larochelle 1994, Engel et al.
2006). An animal can facilitate dry heat dissipation through
various means, but as the ambient temperature increases, conditions for this become less favorable as the gradient between
body temperature and air temperature decreases (Engel et al.
2006). Numerous studies have found that above a threshold
ambient temperature evaporative water loss increases linearly
with temperature (Schleucher et al. 1991, Kvist et al 1998,
Bakken et al. 2002, Engel et al. 2006).
Evaporation can occur from the respiratory tract, eyes,
and cutaneous body surfaces. Panting is a well-documented
behavior for respiratory water evaporation, but the extensive contractions of the respiratory muscles required to do so
are energetically expensive, creating more heat (Arieli et al.
2002). In heat-acclimated birds, cutaneous water evaporation
is particularly important and is enhanced through an increase
in microvessel permeability as a function of the activation of
the transduction pathway for adrenergic signals (Arieli et al.
1999, 2002, Ophir et al. 2004). Although the extent to which
other birds use cutaneous water evaporation is not well understood, in comparison to respiratory water evaporation this
mode of heat dissipation saves energy and reduces heat shock
(Schleucher et al. 1991, McKechnie and Wolf 2004). It is unclear whether either of these processes contribute to heat balance in kestrels. We have observed perched kestrels panting
but do not know the degree to which they may open the bill
during flight to facilitate respiratory evaporation. Although
the cere and lores may provide enhanced surface area for cutaneous water evaporation, it is unknown whether the kestrel
uses this mode of evaporation as extensively as some desertdwelling, heat-acclimated birds do (Arieli et al. 1999).
Body mass and size
Our results indicate that males of larger structural size are
subject to a higher degree of deuterium enrichment (Fig. 3).
562 JENNIFER L. GREENWOOD
and
RUSSELL D. DAWSON
We detected no significant relationship between either size or
mass and δDf in females. Metabolism is proportional to mass;
smaller birds tend to have more moderate metabolism (Bakken et al. 2002), generating less heat. Bakken et al. (2002)
showed that in scolopacid chicks evaporative water loss was
proportional to mass and that the positive relationship between this loss and air temperature was dependent on mass,
likely because the effectiveness of dry heat transfer is dependent on size. Larger kestrels may use evaporative water loss
more because higher energy requirements produce more heat,
which needs to be dissipated. In addition, cutaneous water
evaporation is proportional to surface area, which in turn is
proportional to mass (Bakken et al. 2002), so larger kestrels
with more surface facilitating evaporative water loss may be
subject to greater deuterium enrichment. Alternately, relative surface area decreases with greater mass, and thermal
conductance decreases exponentially with increasing mass
(Schleucher and Withers 2001), so larger-bodied birds may
have a lower capacity for passive heat dissipation and thus
rely more substantially on evaporative water loss. In either
case, if a significant proportion of evaporative water loss is
respiratory, the energy required to pant would itself produce
more heat (Arieli et al. 2002), essentially creating a positive
feedback loop in which larger birds consistently create and
dissipate more heat and lose more depleted hydrogen via evaporative water loss.
Reproductive effort
We found a negative correlation between the volume of a
pair’s clutch and the δDf of the male, suggesting that males
whose females lay clutches of lower volume undergo greater
deuterium enrichment. Although it is not subject to the burden of producing eggs, a male kestrel provides a considerable
amount of food for its mate before laying and during incubation and for the nestlings until they reach an age of ~10 days,
after which more of this responsibility is taken up by the female (Balgooyen 1976). Although it is the female that produces eggs, clutch volume is likely influenced by the amount
of food she receives from her mate prior to and during laying. In our study area, the size of kestrel eggs increased in
response to food supplementation (Wiebe and Bortolotti
1995), and studies of the Common Kestrel (F. tinnunculus)
have shown that variation in food availability drives phenotypic plasticity in clutch size (Korpimäki and Wiehn 1998).
If larger males must devote more time to fulfilling their own
energy demands, this may translate into less time available
for provisioning females and a smaller clutch volume. Our
finding that males of larger size undergo greater deuterium
enrichment could therefore account for the negative relationship between clutch volume and enrichment. This seems unlikely, however, because we found no evidence that larger or
heavier males were mated with females that laid clutches of
smaller volume either within the sample of birds of known
origin (PC1: r = 0.20, P = 0.74, n = 5; residual mass: r =
–0.02, P = 0.97, n = 5) or among all nests at our study site, as
in 2008 (PC1: r = 0.06, P = 0.72, n = 44; residual mass: r =
0.11, P = 0.48, n = 44). Alternately, males may realize lower
clutch volume as a function of the costs associated with lower-quality territories, such as increased time spent avoiding predation or lower food availability. If these males work
harder to provide food for their mates, thereby creating more
heat and decreasing the gradient between body and ambient
temperature, they may also lose more water to evaporation
and hence become enriched in deuterium.
If all else were equal, producing larger nestlings should
require greater energy input by the parent, entailing production of more heat, although many individually variable traits
could uncouple this relationship. Engel et al. (2006) found
that regardless of the ambient temperature or speed of flight,
Rosy Starlings (Sturnus roseus) consistently lost water during flight. Although falcons employ modes of flight different
from those of starlings, and the number of flights depend on
the type of prey being delivered (biomass per trip), if greater
provisioning effort requires more flights to and from the nest,
evaporative water loss should increase. We propose that this
may be related to the positive correlation we observed between
δDf enrichment of adult females and mass of nestling females.
Finally, if as suggested by the heat-dissipation-limit theory,
energy expenditure is limited not by energy supply but by an
individual’s ability to dissipate metabolic heat (Speakman and
Krol 2010), then adults with plentiful access to resources may
be at the upper limit of their ability to dissipate heat and so losing water through evaporation at higher rates; the availability
of these resources may be reflected in nestlings that fledge at
larger sizes.
Conclusions
We have shown that substantial deuterium enrichment occurs
in adult kestrels in relation to the δD of plasma, nestling feathers, and local precipitation, and that the degree of enrichment
is associated with measures of size and reproductive effort at
the time of feather growth. These results lend support to the
evaporative-cooling hypothesis, although we suspect that low
statistical power prevented the detection of further relevant
relationships with some of the variables that we tested. We
did not examine some of the factors that are likely to interact
with those tested here to influence deuterium enrichment. For
example, the degree of evaporative water loss is dependent on
air temperature and the water-vapor deficit between body surfaces and surrounding air (Engel et al. 2006), neither of which
we were able to quantify. In addition, we did not account for
the proportion of time spent incubating by each sex and preexisting conditions that might influence the energy budgets of
adults. Finally, further studies should isolate the body water
contained in plasma to compare δD in this source with that of
both non-water plasma and feathers.
DEUTERIUM ENRICHMENT IN THE AMERICAN KESTREL 563
acknowledgments
We thank K. A. Hobson and two anonymous reviewers for constructive comments on an earlier version of the manuscript. T. J.
Mlynowski assisted with GIS, and C. A. Lott provided helpful discussions during the preliminary stages of data collection. L. E.
Etchart, M. L. Greenwood, C. McKee, and M. Mitsutani provided
valuable assistance in the field. Funding for this research was provided to RDD by the Natural Sciences and Engineering Research
Council of Canada, Canada Foundation for Innovation, British
Columbia Knowledge Development Fund, and the University of
Northern British Columbia, and to JLG by the Northern Scientific
Training Program. This research was approved by the University of
Northern British Columbia Animal Care and Use Committee, and
the data were collected under permit from Environment Canada and
Saskatchewan Ministry of Environment.
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